The present invention relates to methods and compositions useful in subterranean applications, and, more specifically, to stimuli-degradable gels.
Viscosified treatment fluids that are used in subterranean operations generally are often aqueous-based fluids that comprise gelling agents. Viscosified treatment fluids are often referred to in the oilfield industry as “gels.” The term “gel” as used herein refers to a semi-solid, jelly-like state assumed by some colloidal dispersions. The term “colloidal dispersion” as used herein refers to a system in which finely divided particles are dispersed within a continuous medium. The gelling agents used to form gels often comprise macromolecules such as biopolymers or synthetic polymers. Common gelling agents include, e.g., galactomannan gums, cellulosic polymers, and other polysaccharides. As used herein, the term “treatment fluid” refers to any fluid that may be used in a subterranean application in conjunction with a desired function and/or for a desired purpose. The term “treatment fluid” does not imply any particular action by the fluid or any component thereof.
Most viscosified treatment fluids include cross-linked gelling agents that are cross-linked through a cross linking reaction between gelling agent molecules and a suitable cross linking agent. These cross linking agents may comprise a metal, a metal complex, or a metalloid, collectively referred to herein as “metal(s).” Examples include compounds containing boron, aluminum, antimony, zirconium, magnesium, or titanium. Generally, the metal of a cross linking agent interacts with at least two gelling agent molecules to form a crosslink between them, thereby forming a cross-linked gelling agent. The term “cross-linked gelling agent” as used herein refers to a gelling agent that contains, on average, at least one crosslink per molecule. This may be indicated when G′>G″ at certain frequencies. The elastic modulus (or G′) of a gel is an accepted standard measure of a gel's elasticity.
Pills are often used in subterranean applications. The term “pill” as used herein refers to a relatively small volume of specially prepared fluid placed or circulated in the well bore. Fluid pills are commonly prepared for a variety of special functions, such as a sweep pill prepared at high viscosity to circulate around the well bore and pick up debris or well bore fill. In counteracting lost-circulation problems, a lost-circulation pill prepared with flaked or fibrous material is designed to plug the perforations or formation interval losing the fluid. A “fluid-loss control pill” is a gelled fluid that is designed or used to provide some degree of fluid-loss control. Through a combination of viscosity, solids bridging, and cake buildup on the porous rock, these pills oftentimes are thought to seal off portions of the formation from fluid loss. They may also generally enhance filter-cake buildup on the face of the formation to inhibit fluid flow into the formation from the well bore. Pills often may involve a relatively small quantity (less than 200 bbl) of a special blend of a drilling fluid to accomplish a specific task that a regular drilling fluid cannot perform. Examples include high-viscosity pills to help lift cuttings out of a vertical well bore; freshwater pills to dissolve encroaching salt formations; pipe-freeing pills to destroy filter cake and relieve differential sticking forces; and lost circulation material pills to plug a thief zone.
Typically, pills comprise an aqueous base fluid and a high concentration of a gelling agent polymer, and, sometimes, bridging particles, like graded sand, potassium salts, or sized calcium carbonate particles. An example of a commonly used pill contains high concentrations (100 to 150 lbs/1000 gal) of a modified hydroxyethylcellulose (“HEC”). Some other gelling agent polymers that have been used include guar, guar derivatives, carboxymethylhydroxyethylcellulose (“CMHEC”), and even starch.
As an alternative to linear polymeric gels for pills, cross-linked gels often are used. Cross linking the gelling agent polymer is thought to create a gel structure that is better able to support solids and possibly, e.g., provide fluid-loss control. Further, cross-linked pills are thought to invade the formation face to a lesser extent to be desirably effective. To crosslink these gelling agents, a suitable cross linking agent that comprises polyvalent metal ions is often used. Complexes of aluminum, titanium, boron, and zirconium are common examples.
A disadvantage associated with conventional cross-linked gelling agents is that the resultant gel residue is often difficult to remove from the subterranean formation once the treatment has been completed. For example, in fracturing treatments, the cross-linked gels used are thought to be difficult to completely clean up with conventional breakers, such as oxidizers or enzymes. Similarly, the gel residue can be difficult and time-consuming to remove from the subterranean formation. The gel residue, at some point in the completion operation, usually should be removed to restore the formation's permeability, preferably to at least its original level. If the formation permeability is not restored to its original level, production levels can be significantly reduced. This gel residue often requires long cleanup periods. Moreover, an effective cleanup usually requires fluid circulation to provide high driving force, which is thought to allow diffusion to take place to help dissolve the concentrated buildup of the gel residue. Such fluid circulation, however, may not be feasible. Additionally, in lower temperature wells (i.e., those below about 80° F.), it is often difficult to find an internal breaker for the viscosified treatment fluids that will break the gel residue effectively. The term “break” (and its derivatives) as used herein refers to a reduction in the viscosity of the viscosified treatment fluid, e.g., by the breaking or reversing of the crosslinks between polymer molecules or some reduction of the size of the gelling agent polymers. No particular mechanism is implied by the term. Another conventional method of cleaning up gel residue is to add a spot of a strong acid (e.g., 10% to 15% hydrochloric acid) with coiled tubing, which is expensive and can result in hazardous conditions.
New developments in cleaning and removing filter cakes left by fluid loss control additives and pills include materials that degrade under acidic conditions such as calcite. While such techniques can be effective, they require good contact between the acid generating compound and the acid soluble compound, which is not always easily achieved.
Another problem presented by today's cross-linked gelling agent systems with respect to cleanup is that the high temperature of the formations (e.g., bottom hole temperatures of about 200° F. or greater) often require cross linking agents that are more permanent, and thus harder to break. Examples include transition metal cross linking agents. These more permanent cross linking agents can make cleanup of the resulting gel residue more difficult.